For now anyway, although I'm afraid I just jinxed myself.
After several tries, my latest attempt at upgrading the memory in my PowerBook appears to have been successful. Which is good, because (as Artie suggested in the comments) my next step would've been to take the machine to an Apple Store and demand they replace the logic board. Which would've turned into a lengthy process that I wouldn't really want to deal with.
I found this and figured I'd give it a try. The memory arrived today and my machine has been buzzing along for four and a half hours with no problems. Knock on wood.
I think I love my PowerBook again.
